S-Doped Graphene Composites

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In-situ synthesis of CuCo2S4@N/S doped graphene composites with pseudocapacitive properties for high performance lithium ion batteries Pengxiang Wang, Yu Zhang, Yanyou Yin, Lishuang Fan, Naiqing Zhang, and Kening Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00632 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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ACS Applied Materials & Interfaces

In-situ synthesis of CuCo2S4@N/S doped graphene composites with pseudocapacitive properties for high performance lithium ion batteries Pengxiang Wang, † Yu Zhang, † Yanyou Yin, † Lishuang Fan, *, ‡, § Naiqing Zhang, *, ‡, § and Kening Sun‡, § † School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China ‡ State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China § Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, China. E-mail: [email protected], [email protected] Fax: +86-451-86412153; Tel: +86-451-86412153

KEYWORDS: CuCo2S4, pseudocapacity, doped graphene, lithium ion battery, high rate ABSTRACT: To satisfy the demand of high power application, lithium ion batteries with high power density have gained extensive research effort. The pseudocapacitive storage of lithium ion batteries is considered to offer high power density through fast faradic surface redox reactions rather than the slow diffusion controlled intercalation process. In this work, CuCo2S4 anchored

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on N/S doped graphene is in-situ synthesized and a typical pseudocapacitive storage behavior is demonstrated when applied in lithium ion battery anode. The pseudocapacitive storage and N/S doped graphene enable the composite to display a capacity of 453 mAh g-1 after 500 cycles at 2 A g-1, and ultrahigh rate capability of 328 mAh g-1 at 20 A g-1. We believe this work could further promote the research on pseudocapacitive storage in transition metal sulfides for lithium ion battery.

1. INTRODUCTION The rapid development of electric vehicles requires lithium ion batteries (LIBs) with high energy density and excellent rate capability, thus providing extended distance per charge and decreased recharge time for electric vehicles. Recently, transition metal oxides (TMOs) have gained extensive research interest for their higher theoretical capacity than commercial graphite anodes, which could meet the required high energy density of electric vehicles.1-4 However, TMOs usually suffered from large volume variation upon lithiation/ delithiation process, resulting in the pulverization of electrode and unsatisfied cycling performance. Except the large volume variation, the application of TMOs is also restricted by their intrinsic poor conductivity, which causes the unsatisfactory rate capability. Therefore, it is still of great significant to develop new electrode materials with satisfactory cycling performance and rate capability. Transition metal sulfides (TMSs) usually exhibit higher conductivity than their corresponding TMOs5-6 and numerous TMSs such as MoS2,7-10 SnS2,11-12 CoS2,13 and SnS14 et al. have been investigated as anode materials. Ternary TMSs exhibit higher electrochemical activity than binary TMSs for their rich constituent and redox reactions. Recently, ternary TMSs such as NiCo2S4 have also been reported in lithium ion batteries15-17. Fabricating anode materials with


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carbon matrix (graphene, carbon nanotubes, and ordered mesoporous carbon) is an effective strategy to enhance their conductivity and mitigate the volume variation.18-20 Although graphene and other carbon materials possess higher conductivity, they still suffered from low volume energy density because of their low tap density. Beside enhancing the volume energy density, the incorporation of nanoparticles could impede the stacking of graphene sheets, thus exhibiting better cycling stability than pure graphene. In this work, CuCo2S4 nanoparticles are anchored on N/S doped reduced graphene oxide (N/SrGO) through a facile solvothermal method (denoted as CuCo2S4@N/S-rGO, Scheme 1). As lithium storage material, CuCo2S4@N/S-rGO exhibits remarkable cycling stability (a capacity of 453 mAh g-1 at 2 A g-1 after 500 cycles) and superior rate capability with a capacity of 328 mAh g-1 at 20 A g-1, which has never been reported in TMSs at such high rate. The excellent cycling performance could be attributed to the uniform distribution of CuCo2S4 which alleviates the volume variation and impedes the aggregation.21-22 The heteroatom doping has been proved to be effective to enhance the conductivity and electrochemical activity of carbon materials.23-24 More importantly, the distribution of nanoparticles and high conductivity of N/S doped graphene induce the composites with a typical pseudocapacitive storage behavior. Compared with the slow diffusion-controlled lithiation/ delithiation process in traditional electrode materials, the materials with pseudocapacitive storage behavior could deliver higher rete capability as the electrical energy is stored through non-Faradaic electric double layers in the electrode interfaces. It is believed that this work could further promote the research on pseudocapacitive storage in TMSs for LIBs. Scheme 1. The synthesis procedure of CuCo2S4@N/S-rGO composite.


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Cu2+, Co2+, TAA

nucleation

GO

growth

CuCo2S4 nuclei on rGO

CuCo2S4@N/S-rGO

2. EXPERIMENTAL SECTION 2.1 Synthesis of CuCo2S4@N/S-rGO composites, CuCo2S4 and N/S-rGO. GO was synthesized from graphite flakes with a modified Hummers method.25 CuCo2S4@N/SrGO were prepared through a facile solvothermal method. In a typical method, 0.6 mmol Co(CH3COO)2·4H2O and 0.3 mmol Cu(CH3COO)2·H2O were dissolved in 60 mL ethylene glycol. Then 50 mg GO was dispersed in the solution by sonication for 1 h. After stirring for 30 min, 2 mmol thioacetamide was added and stirred at 80 ℃ for 2 h. Subsequently, the mixture was sealed in a 100 mL Telfon-lined autoclave for solvothermal at 200 ℃ for 10 h. After cooling down to room temperature, the product was collected and washed with deionized water and alcohol several times and freeze dried. Pure CuCo2S4 were synthesized with a similar method above, except the addition of GO. N/S-rGO were synthesized with a similar method above, except the addition of Co(CH3COO)2·4H2O and Cu(CH3COO)2·H2O. 2.2 Materials characterizations X-ray diffraction (XRD, PANalytical X’Pert PRO) was performed to analyze the crystal structure of samples. Scanning electron microscopy (SEM, Hitachi Limited SU-8010) and transmission electron microscopy (TEM, Hitachi Limited H7650) were used to characterize the morphology of as-prepared samples. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha) was used to characterize the element composition and valence state of


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samples. The specific surface and pore structures of samples were analyzed through Nitrogen adsorption-desorption measurements (Micromeritics ASAP2020). 2.3 Electrochemical measurements. The electrochemical performance of CuCo2S4@N/S-rGO and CuCo2S4 were evaluated as anode materials in half cells. Super P and polyvinylidene fluoride (PVDF) were used as conductive agent and binder respectively, which were mixed with CuCo2S4 at a weight ratio of 5: 5: 90 in N-methyl-2-pyrrolidinone (NMP) solvent. CuCo2S4@N/S-rGO was mixed with PVDF at a weight ratio of 95: 5. Then the mixture was coated on the surface of copper foil and dried in vacuum oven for 12 h at 120 ℃. The foil was shaped into a circular pellet with a diameter of 12 mm and pressed to enhance the contact between the active material and Cu foil. The mass loading was about 1 mg cm-2. The electrochemical properties of CuCo2S4@N/S-rGO and CuCo2S4 were measured in half cells. In a coin type cell (CR2025), a lithium foil was used as counter electrode and separated to work electrode with a piece of porous polypropylene films (Celgard 2400). The cells were assembled in an argon-filled glove box using 1 M LiPF6 in diethyl carbonate, ethylene carbonate and ethyl methyl carbonate (DC/EC/EMC, 1: 1: 1 by vol) as an electrolyte. Charge/discharge measurements were carried out with Neware battery test systems in a voltage range of 0.01-3.0 V (vs. Li/Li+) at various current rates. Electrical impedance spectroscopy (EIS) measurements were performed in a frequency range from 100 kHz to 100 mHz using a PARSTAT 2273 advanced electrochemical system. Cyclic voltammetry (CV) measurements were carried at a scan rate of 0.1 mV s-1 in a potential range from 0.01 V to 3.0 V (vs. Li/Li+) with a CHI-650 electrochemical work station.

3. RESULTS AND DISCUSSION


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XRD patterns of CuCo2S4@N/S-rGO and CuCo2S4 are shown in Figure 1a, and all peaks are consistent with the standard card of the Joint Committee on Powder Diffraction Standards (JCPDS No. 09-0425). The characteristic peak of graphene layers around 2θ = 25° could not be detected in CuCo2S4@N/S-rGO, which may be attributed to the distribution of CuCo2S4 nanoparticles on graphene sheets and impede their stack. SEM images of CuCo2S4@N/S-rGO are shown in Figure 1b, which indicates that CuCo2S4 nanoparticles are uniformly anchored on the surface of graphene. For comparison, CuCo2S4 obtained from the absence of GO were observed aggregated together (Figure S1), indicating that the graphene could prevent the aggregation of CuCo2S4 particles. TEM images of CuCo2S4@N/S-rGO are presented in Figure 1c, which further indicates that CuCo2S4 nanoparticles were hybridized with graphene. Figure 1d shows the inter-planar spacing of 0.345 nm, which can be attributed to the (022) plane of CuCo2S4@N/S-rGO. The SEM and TEM images provide clear information that CuCo2S4 nanoparticles and graphene were hybridized together.


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Figure 1. a) XRD patterns of CuCo2S4@N/S-rGO and CuCo2S4. b) SEM images of CuCo2S4@N/S-rGO. c, d) TEM images of CuCo2S4@N/S-rGO. XPS spectra was used to characterize the surface element composition and valence state of CuCo2S4@N/S-rGO. As shown in the XPS survey spectra of CuCo2S4 (figure S2a) and CuCo2S4@N/S-rGO (figure S2b), peaks of Cu 2p, Co 2p, and S 2p are clearly observed. Figure 2 shows the typical fitted Cu 2p spectra, Co 2p, S 2p and N 1s spectra of CuCo2S4@N/S-rGO. As shown in Figure 2a, the spectra of Cu 2p are constituted with peaks of 2p3/2 (932.0 eV), 2p1/2(952.0 eV) and two satellite peaks. In the XPS spectra of Co 2p, the 2p3/2 and 2p1/2 were both deconvoluted into two peaks, which can be ascribed to Co3+ (778.5eV and 793.5 eV) and Co2+ (780.4 eV and 797.6 eV), respectively.26 The S 2p peaks were deconvoluted into a satellite peak (168.7 eV) and three peaks. The peaks at 161.6 eV and 162.7 eV are ascribed to S 2p3/2 and S 2p1/2 respectively. The peak at 164.1 eV suggested the presence of sulfur-metal (S-M) bond， which was also observed in the S 2p spectra of CuCo2S4 and not in N/S-rGO (Figure S3a and b).27 Therefore, the XPS results further confirmed the composition of CuCo2S4@N/S-rGO. Figure 2d exhibits the N 1s spectra of CuCo2S4@N/S-rGO with well fitted three peaks at 399.0 eV, 399.9 eV and 401.5 eV, which can be assigned to pyridine-type N, pyrrole-type N and graphitic N.28-30 The N 1s spectra is also observed in N/S-rGO (Figure S3c). This result confirmed that the graphene is N doped.


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Figure 2. High resolution XPS spectra of (a) Cu 2p, (b) Co 2p, (c) S 2p and (d) N 1s for CuCo2S4@N/S-rGO. To evaluate the structural feature and porosity of obtained CuCo2S4@N/S-rGO and CuCo2S4, Brunauer-Emmett-Teller (BET) analysis was carried out. The nitrogen adsorption-desorption isotherms and pore size distribution curves of CuCo2S4@N/S-rGO and CuCo2S4 are shown in Figure 3a and b, and the BET specific surface area is calculated to be 69.5 m2 g-1 for CuCo2S4@N/S-rGO and 44.2 m2 g-1 for CuCo2S4. The nitrogen adsorption-desorption isotherm of CuCo2S4@N/S-rGO found to be matching with type IV BET classification. As shown in the pore size distribution curve, the pores of CuCo2S4@N/S-rGO are mainly mesopores within 50 nm and macropores within 100 nm. On the contrary, there are hardly any mesopores and macropores in CuCo2S4. These mesopores and macropores of CuCo2S4@N/S-rGO mainly arise from the incorporation of CuCo2S4 nanoparticles and graphene sheets.


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Figure 3. Nitrogen adsorption-desorption isotherms and pore size distribution curves of CuCo2S4@N/S-rGO (a) and CuCo2S4 (b). The electrochemical reactions of CuCo2S4@N/S-rGO were investigated through CV tests. In the first cycle of CuCo2S4@N/S-rGO (Figure 4a), the cathodic peaks can be attributed to the insertion of Li+ in CuCo2S4 and the reduction of Cu2+ and Co3+ to metal Cu and Co. The anodic peaks centered at 2.01 V and 2.34 V originate from the formation of CuS and CoS. During the following cycles, the reduction peaks shift to lower potential, also in good agreement with the voltage profiles (Figure S4). The evident peak located at 0.78 V which exists only in the first cathodic scan process of CuCo2S4@N/S-rGO can be ascribed to the formation of solid electrolyte interphase (SEI) film. The almost overlapped CV curves after first cycle indicate good reversibility of the CuCo2S4@N/S-rGO during cycling process. The rate capability of CuCo2S4@N/S-rGO was investigated at various current density (Figure 4b and c). Benefiting from the excellent conductivity of N/S doped graphene and the distribution of CuCo2S4 on graphene, CuCo2S4@N/S-rGO deliveres discharge capacity of 984, 777, 726, 685, 597 and 506 mAh g-1 at 0.2, 0.5, 1, 2, 5 and 10 A g-1 respectively. Even at a high current density of 20 A g-1, it still delivers a reversible capacity of 328 mAh g-1. When the current density recovers to 0.2 A g-1, the capacity returns back to 886 mAh g-1. Furthermore, the similarity of charge and discharge curves of CuCo2S4@N/S-rGO at different current indicates that the


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polarization even at high current density is slight, further demonstrates the excellent rate capability.31-32 On the contrast, the specific capacity of CuCo2S4 decreased dramatically at initial cycles, and only a small specific capacity of 12 mAh g-1 was remained at 5 A g-1. Although capacities of some reported carbon materials decrease slighter than CuCo2S4@N/S-rGO composites with the increase of current densities, the specific capacities of obtained CuCo2S4@N/S-rGO at high current densities are higher than other metal sulfides and carbon materials in the reported literatures (Figure S5a and b). The excellent electrochemical performance could be attributed to the uniform distribution of CuCo2S4 which alleviates the volume variation and impeding the aggregation, and the high conductivity of N/S-rGO. The cycling stability of CuCo2S4@N/S-rGO and CuCo2S4 were tested at a current density of 1 A g-1 (Figure 4d). After 30 cycles, the capacity of CuCo2S4@N/S-rGO slightly drops to 545 mAh g-1 and then increases to 708 mAh g-1 after 150 cycles. Compared with CuCo2S4@N/S-rGO, CuCo2S4 exhibits much inferior cycling stability with a discharge capacity dramatically drops less than 100 mAh g-1 after 31 cycles. When cycled at a higher current density of 2 A g-1, CuCo2S4@N/S-rGO still displays favorable cycling stability with a discharge capacity of 453 mAh g-1 after 500 cycles (Figure 4e).


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1.0

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Figure 4. a) CV curves of CuCo2S4@N/S-rGO during the initial four cycles at 0.1 mV s-1. b) Charge-discharge curves of CuCo2S4@N/S-rGO at different current density. c) Rate capability of CuCo2S4@N/S-rGO and CuCo2S4. d) Cycling stability of CuCo2S4@N/S-rGO and CuCo2S4 at 1 A g-1. e) Cycling stability of CuCo2S4@N/S-rGO at 2 A g-1. The ultrahigh rate capability of CuCo2S4@N/S-rGO could not be explained by the lithiation/delithiation process, which is diffusion controlled and slow. Considering the nanosized CuCo2S4 and high conductivity of N/S-rGO, the ultrahigh rate capability might could be attributed to the pseudocapacitive effect, which is surface controlled. To characterize the


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contribution of pseudocapacitive storage, CV tests at different scan rates from 1 to 12 mV s-1 were carried out (Figure 5a). The relationship between current (i) and scan rate (v) obey the power law.30, 33 i = avb

(1)

The b value can be determined by the slope of the log(i) ~ log(v) plot (Figure 5b), which reflect the control process of electrochemical reactions. A b value of 1.0 or 0.5 represents the electrochemical reaction is surface or diffusion controlled respectively. According to the log(i) ~ log(v) plots at 0.9 V and 1.9 V, the b value is 1.064 and 0.924 respectively, indicating the electrode process is surface controlled. The peak current(Ip) could be further separated into a capacitive effect (k1v) and a diffusion controlled process (k2v0.5), which represent the contribution of pseudocapacitive storage and diffusion controlled process respectively.34 Ip = k1v + k2v0.5

(2)

The equation (2) could be further modified as equation (3). Ip/v0.5 = k1v0.5 + k2

(3)

According to the plot of Ip/v0.5 ~ v0.5, k1 and k2 were calculated to be 0.358 and 0.008 respectively (Figure 5c). Figure 5d displays the contribution of capacitive at different scan rates, which is 58.6% at 1 mV s-1 and increase to 83.1% at 12 mV s-1 gradually. The CV curves at high scan rates from 14 to 30 mV s-1 are shown in figure S6. The contribution of capacitive at 30 mV s-1 which corresponding to a current density of 20 A g-1 is calculated to be 88.6%. The b value and capacitive contribution demonstrate that the major capacity of CuCo2S4@N/S-rGO comes from the pseudocapacitive storage, which is beneficial for high rate capability and cycling performance. The pseudocapacitive storage behavior of CuCo2S4@N/S-rGO can be attributed to the distribution of nanoparticles and high conductivity of N/S-rGO.


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10 8

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Figure 5. a) CV curves of CuCo2S4@N/S-rGO at different scan rates. b) log (i) ~ log (v) plots at 0.9 V and 1.9 V. c) Ip/v0.5 ~ v0.5 plot of CuCo2S4@N/S-rGO. d) Contribution ratio of capacitive at different scan rate. Electrochemical impedance spectroscopy measurements (EIS) were carried out to understand the reasons for the impressive chemical performance of CuCo2S4@N/S-rGO. The Nyquist plots of CuCo2S4@N/S-rGO and CuCo2S4 both display a high-frequency semicircle and an inclined line, which corresponding to the charge-transfer resistance (Rct) and the lithium-diffusion process (Figure 6a). The CuCo2S4 shows an obvious Rct of 629 Ω. In comparison, the Rct value of CuCo2S4@N/S-rGO composite is only 228 Ω, implying the higher conductivity of the composite and rapid charge transfer process for the electrochemical reactions. Compared with initial CuCo2S4@N/S-rGO, the Nyquist plots after discharge, recharged and cycles are composed of two depressed semicircles and an inclined line. The additional semicircle is related to the impedance of SEI film.35 The Nyquist plots indicate that the impedance decreased after


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discharge, which can be attributed to the formation of metal Cu and Co during discharge process. Although the impedance of CuCo2S4@N/S-rGO increased after 100 cycles at 1A g-1, it is still much lower than initial CuCo2S4. This result further demonstrates the higher conductivity of the composite during cycling process. The SEM images of CuCo2S4@N/S-rGO discharged and recharged after 100 cycles are also provided in figure 6 c and d. Although CuCo2S4@N/S-rGO is slightly vague as covered by the binder and SEI film, the nanoparticles of CuCo2S4 are still could be clearly observed and not agglomerated, which indicate the structural stability of the composite during the electrochemical process.

Figure 6. a) Nyquist plots of CuCo2S4@N/S-rGO and CuCo2S4; b) Nyquist plots of CuCo2S4@N/S-rGO after discharge, after recharged and after 100 cycles at 1A g-1; SEM images of CuCo2S4@N/S-rGO discharged (c) and after recharged (d) after 100 cycles.

4. CONCLUSION


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In summary, CuCo2S4 anchored on N/S doped graphene is in-situ synthesized. The distribution of CuCo2S4 nanoparticles and high conductivity of N/S-rGO provide CuCo2S4@N/S-rGO with better stability and high conductivity, thus exhibiting better cycling and rate performance than pure CuCo2S4. Moreover, the capacitive contribution was calculated and pseudocapacitive storage behavior of CuCo2S4@N/S-rGO was demonstrated, which is beneficial for high rate capability and cycling performance. The CuCo2S4@N/S-rGO exhibits excellent cycling stability with a capacity of 453 mAh g-1 after 500 cycles at 2 A g-1 and superior rate capability with 328 mAh g-1 at 20 A g-1.

ASSOCIATED CONTENT Supporting Information. Additional characterization information, including SEM images, XPS patterns, and electrochemical performance details. AUTHOR INFORMATION Corresponding Author *(L. F.) [email protected] *(N. Z.) [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS


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This work was supported by the National Natural Science Foundation of China (no. 21646012), the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (no. 2016DX08). China Postdoctoral Science Foundation (no. 2016M600253, 2017T100246), and the Postdoctoral Foundation of Heilongjiang Province. The Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 201836).

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S-Doped Graphene Composites

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